1. Field of the Invention
The present invention relates to a digital image forming apparatus such as a digital copying machine which forms an electrostatic latent image with optical intensity modulation.
2. Description of the Prior Art
In a digital image forming apparatus, a laser beam is modulated to form an electrostatic latent image on a photoconductor. Optical intensity modulation technique is one modulation technique for forming an electrostatic latent image with a laser beam. In the optical intensity modulation technique, an optical intensity of a laser beam for exposing a photoconductor is modulated according to image data of each pixel. Because modulation is performed in the unit of a pixel, a density is expressed for an image data of one pixel, and both gradation and resolution of a reproduced image are good.
However, it is difficult to stabilize gradation reproduction in a high-lighted portion in an image where delicate density control is needed. Further, granularity at a half-tone portion is liable to become worse due to random noises. It is proposed to decrease duty ratio of light-emitting time of laser beam less than 100% in the optical intensity modulation technique in order to improve granularity and stability of gradation reproduction. However, it is disadvantageous that resolution is deteriorated by fine lines in an image due to non-light-emitting periods.